Cubic system boron nitride, hereinafter referred to simply as cBN, has a hardness next to that of diamond and it has excellent thermal and chemical stability characteristics. Therefore, cBN has atracted special interest as a material for working tools. In addition, cBN has a high heat conductivity also next to that of diamond and accordingly, it is expected to be used for purposes such as the material of a heat-radiation substrate.
As the material of a heat-radiation substrate, various materials having characteristics as shown in Table 1 have been used conventionally.
TABLE 1 ______________________________________ Characteristics Thermal dielectric Heat Expansion constant Conductivity Coefficient (1 MHz Resistivity (at room (at room at room (at room temperature) temperature temper- temperature) Material W/cm .multidot. .degree.C. to 400.degree. C. ature) .OMEGA. .multidot. cm ______________________________________ SiC 2.7 3.7 45 10.sup.13 BeO 2.6 7.6 6.about.8 10.sup.14 AlN 0.6.about.1.6 4.0 8 10.sup.12 Al.sub.2 O.sub.3 0.2 6.7 8.about.10 10.sup.14 Si 1.3 3.6 12 10.sup.-3.about.3 Diamond 20 2.3 5.7 10.sup.16 ______________________________________
Table 1 shows that diamond has a very high heat-conductivity compared with the other materials.
On the other hand, Slack predicted in J. Phys. Chem. Solids, Vol. 34 (1972)pages 321 to 334 that pure single crystal cBN would have a heat-conductivity as high as approximately 13 W/cm.degree.C. at room temperature and suggested the possibility of using it as the material for a radiation substrate.
However, a large-size single crystal of cBN has not yet been produced as far as we know, and accordingly, the heat conductivity of 13 W/cm.degree.C. has not been confirmed for cBN.
In addition, the largest value reported up to the present as the heat conductivity of a cBN sintered body containing a binding phase is only 2 W/cm..degree.C. The reason for this is supposed to be that the binding phase acts as an important factor of phonon scattering, causing the heat conductivity to be lowered excessively. Heat conductivity in non-metallic electrically insulating crystals is directly proportional to the phonon mean free path, see U.S. Pat. No. 4,188,194 (Corrigan), issued Feb. 12, 1980, col. 19.
Corrigan discloses a method for manufacturing a high heat-conductivity cBN sintered body without a binding phase, wherein a high density cBN sintered body having a high heat conductivity within the range of about 3 W/cm.degree.C. to about 9 W/cm.degree.C. is manufactured by a direct conversion process using pyrolytic boron nitride (pBN) as the starting raw material, please see FIG. 16 of Corrigan. Where Corrigan starts with hexagonal boron nitride in his Examples 29 and 30 the obtained heat conductivity is only about 1.33 W/cm..degree.C. or 1.07 W/cm.degree.C., respectively. In col. 22, Corrigan states: "The room temperature thermal conductivity of the best U-PBN compacts is higher by a factor of 6-8 compared to the directly converted HBN powder compact (Example 29) and by a factor of about 10 compared to the composite compact (Example 30).". This result does not suggest, nor does it motivate the use of hBN as a starting material if one wants to obtain cBN with a high thermal conductivity.
Further, in the method of the U.S. Pat. No. 4,188,194 (Corrigan), a very high pressure of about 7 GPa and high temperatures of 2000.degree. C. or more are required to manufacture a sintered body having a heat-conductivity of 4 W/cm..degree.C. or more. There is also a problem in that the results disclosed by Corrigan are not consistently reproducible. In addition, pyrolytic BN is a very expensive material.
On the other hand, a method for manufacturing a cBN sintered body not containing a binding phase under relatively mild conditions and at low cost is disclosed for example in a paper by Wakatsuki et al. in "Mat Res Bull" Vol 7 (1972) page 999, in which a cBN sintered body is obtained by a direct conversion process using hexagonal system boron nitride having a low degree of crystallization. However, the hBN of low degree of crystallization used as the starting raw material by Wakatsuki et al. lacks chemical stability and is liable to react with the oxygen in the air, which makes it difficult to obtain a homogenous body uniformly and sufficiently sintered overall.
The inventors of the present invention have conducted experiments on synthetic materials using various methods in order to manufacture a sintered cBN body having a high thermal conductivity of at least 4 W/cm.degree.C. and at a low cost while permitting a good reproductiveness with consistent results. As a result, they found it most suitable to use methods as disclosed in U.S. Pat. No. 4,469,802 (Endo et al.), issued on Sep. 4, 1984, where boron nitride of an alkaline earth metal or alkali metal is mixed or diffused into a hexagonal boron nitride (hBN) and then the material is subjected to a high temperature of 1350.degree. C. or above under a thermodynamically stabilized pressure condition of cBN. Endo et al. intend to provide a sintered cBN body having a good light-transmitting property and are not concerned with obtaining a sintered cBN body having a good heat conductivity. Part or all of the added hBN is diffused and removed out of the system in the above stated Endo et al. method at the time of sintering under a high pressure, whereby a sintered body comprised of substantially 100% cBN can be obtained.
As a result of measuring the heat conductivity of a sintered body obtained as described above according to Endo et al., it was found that the heat conductivity of such a sintered body had a relatively high value of 2 to 3 W/cm..degree.C. on the average compared with other sintered bodies using binding materials. However, the heat conductivity measurements also showed that in some cases the heat conductivity of such a sintered body was as low as 1.7 W/cm..degree.C. and thus the measured values were scattered in a rather wide range.
French Patent Publication 2,344,642 (Lalaurie et al.), published Oct. 14, 1977 discloses a process for realizing metal deposits on supports of boron nitride. Lalaurie et al. use the type of boron nitride disclosed in an article entitled "Chemical Vapor Deposited Materials for Electron Tubes" by S. R. Steele et al., published by "Clearinghouse for Federal Scientific and Technical Information" Springfield, VA, 22151, Apr. 1969, No: AD 686,342, pages 29 and 43 to 48. According to page 29 of the Steele et al. disclosure, the isotropic CVD boron nitride (standard grade) has a thermal conductivity of 0.188 W/cm..degree.C. (=0.045 cal/cm.sup.2 /cm/sec/.degree.C. at 300.degree. C.). Lalaurie et al. do not mention anything regarding the thermal conductivity their boron nitride substrate should have.